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@ -12,10 +12,8 @@ behind it are, and it gets really ugly really fast as soon as you try to mix it
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with any other ordering.
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To understand `SeqCst`, we first have to understand the problem it exists to
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solve. The first complexity is that this problem can only be observed in the
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presence of at least four different threads _and_ two separate atomic variables;
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anything less and it’s not possible to notice a difference. The common example
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used to show where weaker orderings produce counterintuitive results is this:
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solve. A simple example used to show where weaker orderings produce
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counterintuitive results is this:
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```rust
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# use std::sync::atomic::{self, AtomicBool};
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@ -29,56 +27,48 @@ const ORDERING: atomic::Ordering = atomic::Ordering::Relaxed;
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static X: AtomicBool = AtomicBool::new(false);
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static Y: AtomicBool = AtomicBool::new(false);
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let a = thread::spawn(|| { X.store(true, ORDERING) });
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let b = thread::spawn(|| { Y.store(true, ORDERING) });
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let c = thread::spawn(|| { while !X.load(ORDERING) {} Y.load(ORDERING) });
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let d = thread::spawn(|| { while !Y.load(ORDERING) {} X.load(ORDERING) });
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let a = thread::spawn(|| { X.store(true, ORDERING); Y.load(ORDERING) });
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let b = thread::spawn(|| { Y.store(true, ORDERING); X.load(ORDERING) });
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let a = a.join().unwrap();
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let b = b.join().unwrap();
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let c = c.join().unwrap();
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let d = d.join().unwrap();
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# return;
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// This assert is allowed to fail.
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assert!(c || d);
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assert!(a || b);
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```
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The basic setup of this code, for all of its possible executions, looks like
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this:
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```text
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a static X c d static Y b
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╭─────────╮ ┌───────┐ ╭─────────╮ ╭─────────╮ ┌───────┐ ╭─────────╮
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│ store X ├─┐ │ false │ ┌─┤ load X │ │ load Y ├─┐ │ false │ ┌─┤ store Y │
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╰─────────╯ │ └───────┘ │ ╰────╥────╯ ╰────╥────╯ │ └───────┘ │ ╰─────────╯
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└─┬───────┐ │ ╭────⇓────╮ ╭────⇓────╮ │ ┌───────┬─┘
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│ true ├─┘ │ load Y ├─? ?─┤ load X │ └─┤ true │
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└───────┘ ╰─────────╯ ╰─────────╯ └───────┘
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a static X static Y b
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╭─────────╮ ┌───────┐ ┌───────┐ ╭─────────╮
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│ store X ├─┐ │ false │ │ false │ ┌─┤ store Y │
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╰────╥────╯ │ └───────┘ └───────┘ │ ╰────╥────╯
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╭────⇓────╮ └─┬───────┐ ┌───────┬─┘ ╭────⇓────╮
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│ load Y ├─? │ true │ │ true │ ?─┤ load X │
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╰─────────╯ └───────┘ └───────┘ ╰─────────╯
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```
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In other words, `a` and `b` are guaranteed to, at some point, store `true` into
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`X` and `Y` respectively, and `c` and `d` are guaranteed to, at some point, load
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those values of `true` from `X` and `Y` (there could also be an arbitrary number
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of loads of `false` by `c` and `d`, but they’ve been omitted since they don’t
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actually affect the execution at all). The question now is when `c` and `d` load
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from `Y` and `X` respectively, is it possible for them _both_ to load `false`?
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In other words, `a` and `b` are guaranteed to store `true` into `X` and `Y`
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respectively, and then attempt to load from the other thread’s atomic. The
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question now is: is it possible for them _both_ to load `false`?
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And looking at this diagram, there’s absolutely no reason why not. There isn’t
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even a single arrow connecting the left and right hand sides so far, so the load
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has no coherence-based restrictions on which value it is allowed to pick — and
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this goes for both sides equally, so we could end up with an execution like
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this:
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even a single arrow connecting the left and right hand sides so far, so the
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loads have no coherence-based restrictions on which values they are allowed to
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pick, and we could end up with an execution like this:
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```text
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a static X c d static Y b
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╭─────────╮ ┌───────┐ ╭─────────╮ ╭─────────╮ ┌───────┐ ╭─────────╮
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│ store X ├─┐ │ false ├┐ ┌┤ load X │ │ load Y ├┐ ┌┤ false │ ┌─┤ store Y │
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╰─────────╯ │ └───────┘│ │╰────╥────╯ ╰────╥────╯│ │└───────┘ │ ╰─────────╯
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└─┬───────┐└─│─────║──────┐┌─────║─────│─┘┌───────┬─┘
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│ true ├──┘╭────⇓────╮┌─┘╭────⇓────╮└──┤ true │
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└───────┘ │ load Y ├┘└─┤ load X │ └───────┘
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╰─────────╯ ╰─────────╯
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a static X static Y b
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╭─────────╮ ┌───────┐ ┌───────┐ ╭─────────╮
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│ store X ├┐ │ false ├─┐┌┤ false │ ┌┤ store Y │
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╰────╥────╯│ └───────┘┌─┘└───────┘ │╰────╥────╯
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║ │ ┌─────────┘└───────────┐│ ║
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╭────⇓────╮└─│┬───────┐ ┌───────┬─│┘╭────⇓────╮
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│ load Y ├──┘│ true │ │ true │ └─┤ load X │
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╰─────────╯ └───────┘ └───────┘ ╰─────────╯
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|
```
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Which results in a failed assert. This execution is brought about because the
|
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|
@ -178,16 +168,16 @@ make that execution invalid. As a refresher, here’s the framework for every
|
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|
|
|
possible execution of the program:
|
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|
|
|
|
|
|
|
```text
|
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|
|
|
a static X c d static Y b
|
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|
|
|
╭─────────╮ ┌───────┐ ╭─────────╮ ╭─────────╮ ┌───────┐ ╭─────────╮
|
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|
|
|
│ store X ├─┐ │ false │ ┌─┤ load X │ │ load Y ├─┐ │ false │ ┌─┤ store Y │
|
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|
╰─────────╯ │ └───────┘ │ ╰────╥────╯ ╰────╥────╯ │ └───────┘ │ ╰─────────╯
|
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|
|
|
└─┬───────┐ │ ╭────⇓────╮ ╭────⇓────╮ │ ┌───────┬─┘
|
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|
|
│ true ├─┘ │ load Y ├─? ?─┤ load X │ └─┤ true │
|
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|
|
└───────┘ ╰─────────╯ ╰─────────╯ └───────┘
|
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|
|
|
a static X static Y b
|
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|
|
|
╭─────────╮ ┌───────┐ ┌───────┐ ╭─────────╮
|
|
|
|
|
│ store X ├─┐ │ false │ │ false │ ┌─┤ store Y │
|
|
|
|
|
╰────╥────╯ │ └───────┘ └───────┘ │ ╰────╥────╯
|
|
|
|
|
╭────⇓────╮ └─┬───────┐ ┌───────┬─┘ ╭────⇓────╮
|
|
|
|
|
│ load Y ├─? │ true │ │ true │ ?─┤ load X │
|
|
|
|
|
╰─────────╯ └───────┘ └───────┘ ╰─────────╯
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
First of all, both the final loads (`c` and `d`’s second operations) need to
|
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|
|
First of all, both the final loads (`a` and `b`’s second operations) need to
|
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|
|
|
become `SeqCst`, because they need to be aware of the total ordering that
|
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|
|
|
determines whether `X` or `Y` becomes `true` first. And secondly, we need to
|
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|
|
establish that ordering in the first place, and that needs to be done by making
|
|
|
|
|
@ -195,77 +185,74 @@ sure that there is always one operation in _S_ that both sees one of the atomics
|
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|
|
as `true` and precedes both final loads in _S_, so that the coherence ordering
|
|
|
|
|
guarantee will apply (the final loads themselves don’t work for this since
|
|
|
|
|
although they “know” that their corresponding atomic is `true` they don’t
|
|
|
|
|
interact with it directly so _S_ doesn’t care).
|
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|
|
There are two operations in the program that could fulfill the first condition,
|
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|
|
should they be made `SeqCst`: the stores of `true` and the first loads. However,
|
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|
|
|
the second condition ends up ruling out using the stores, since in order to make
|
|
|
|
|
sure that they precede the final loads in _S_ it would be necessary to have the
|
|
|
|
|
first loads be `SeqCst` anyway (due to the mixed-`SeqCst` special case detailed
|
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|
|
|
later), so in the end we can just leave them as `Relaxed`.
|
|
|
|
|
interact with it directly so _S_ doesn’t care) — for this, we must set both
|
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|
|
stores to use the `SeqCst` ordering.
|
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|
|
|
|
|
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|
|
This leaves us with the correct version of the above program, which is
|
|
|
|
|
guaranteed to never panic:
|
|
|
|
|
|
|
|
|
|
```rust
|
|
|
|
|
# use std::sync::atomic::{AtomicBool, Ordering::{Relaxed, SeqCst}};
|
|
|
|
|
# use std::sync::atomic::{self, AtomicBool};
|
|
|
|
|
use std::thread;
|
|
|
|
|
|
|
|
|
|
const ORDERING: atomic::Ordering = atomic::Ordering::SeqCst;
|
|
|
|
|
|
|
|
|
|
static X: AtomicBool = AtomicBool::new(false);
|
|
|
|
|
static Y: AtomicBool = AtomicBool::new(false);
|
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|
|
|
|
|
|
|
|
let a = thread::spawn(|| { X.store(true, Relaxed) });
|
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|
|
|
let b = thread::spawn(|| { Y.store(true, Relaxed) });
|
|
|
|
|
let c = thread::spawn(|| { while !X.load(SeqCst) {} Y.load(SeqCst) });
|
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|
|
|
let d = thread::spawn(|| { while !Y.load(SeqCst) {} X.load(SeqCst) });
|
|
|
|
|
let a = thread::spawn(|| { X.store(true, ORDERING); Y.load(ORDERING) });
|
|
|
|
|
let b = thread::spawn(|| { Y.store(true, ORDERING); X.load(ORDERING) });
|
|
|
|
|
|
|
|
|
|
let a = a.join().unwrap();
|
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|
|
|
let b = b.join().unwrap();
|
|
|
|
|
let c = c.join().unwrap();
|
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|
|
|
let d = d.join().unwrap();
|
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|
|
|
|
|
|
|
|
# return;
|
|
|
|
|
// This assert is **not** allowed to fail.
|
|
|
|
|
assert!(c || d);
|
|
|
|
|
assert!(a || b);
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
As there are four `SeqCst` operations with a partial order between two pairs in
|
|
|
|
|
them (caused by the sequenced-before relation), there are six possible
|
|
|
|
|
executions of this program:
|
|
|
|
|
|
|
|
|
|
- All of `c`’s loads precede `d`’s loads:
|
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|
|
|
1. `c` loads `X` (gives `true`)
|
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|
|
|
1. `c` loads `Y` (gives either `false` or `true`)
|
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|
|
|
1. `d` loads `Y` (gives `true`)
|
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|
|
|
1. `d` loads `X` (required to be `true`)
|
|
|
|
|
- Both initial loads precede both final loads:
|
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|
|
|
1. `c` loads `X` (gives `true`)
|
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|
|
|
1. `d` loads `Y` (gives `true`)
|
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|
|
|
1. `c` loads `Y` (required to be `true`)
|
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|
|
|
1. `d` loads `X` (required to be `true`)
|
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|
|
|
- As above, but the final loads occur in a different order:
|
|
|
|
|
1. `c` loads `X` (gives `true`)
|
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|
|
|
1. `d` loads `Y` (gives `true`)
|
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|
|
|
1. `d` loads `X` (required to be `true`)
|
|
|
|
|
1. `c` loads `Y` (required to be `true`)
|
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|
|
|
- As before, but the initial loads occur in a different order:
|
|
|
|
|
1. `d` loads `Y` (gives `true`)
|
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|
|
|
1. `c` loads `X` (gives `true`)
|
|
|
|
|
1. `c` loads `Y` (required to be `true`)
|
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|
|
|
1. `d` loads `X` (required to be `true`)
|
|
|
|
|
- As above, but the final loads occur in a different order:
|
|
|
|
|
1. `d` loads `Y` (gives `true`)
|
|
|
|
|
1. `c` loads `X` (gives `true`)
|
|
|
|
|
1. `d` loads `X` (required to be `true`)
|
|
|
|
|
1. `c` loads `Y` (required to be `true`)
|
|
|
|
|
- All of `d`’s loads precede `c`’s loads:
|
|
|
|
|
1. `d` loads `Y` (gives `true`)
|
|
|
|
|
1. `d` loads `X` (gives either `false` or `true`)
|
|
|
|
|
1. `c` loads `X` (gives `true`)
|
|
|
|
|
1. `c` loads `Y` (required to be `true`)
|
|
|
|
|
- All of `a`’s operations precede `b`’s operations:
|
|
|
|
|
1. `a` stores `true` into `X`
|
|
|
|
|
1. `a` loads `Y` (gives `false`)
|
|
|
|
|
1. `b` stores `true` into `Y`
|
|
|
|
|
1. `b` loads `X` (required to give `true`)
|
|
|
|
|
- All of `b`’s operations precede `a`’s operations:
|
|
|
|
|
1. `b` stores `true` into `Y`
|
|
|
|
|
1. `b` loads `X` (gives `false`)
|
|
|
|
|
1. `a` stores `true` into `X`
|
|
|
|
|
1. `a` loads `Y` (required to give `true`)
|
|
|
|
|
- The stores precede the loads,
|
|
|
|
|
`a`’s store precedes `b`’s and `a`’s load precedes `b`’s:
|
|
|
|
|
1. `a` stores `true` to `X`
|
|
|
|
|
1. `b` stores `true` into `Y`
|
|
|
|
|
1. `a` loads `Y` (required to give `true`)
|
|
|
|
|
1. `b` loads `X` (required to give `true`)
|
|
|
|
|
- The stores precede the loads,
|
|
|
|
|
`a`’s store precedes `b`’s and `b`’s load precedes `a`’s:
|
|
|
|
|
1. `a` stores `true` to `X`
|
|
|
|
|
1. `b` stores `true` into `Y`
|
|
|
|
|
1. `b` loads `X` (required to give `true`)
|
|
|
|
|
1. `a` loads `Y` (required to give `true`)
|
|
|
|
|
- The stores precede the loads,
|
|
|
|
|
`b`’s store precedes `a`’s and `a`’s load precedes `b`’s:
|
|
|
|
|
1. `b` stores `true` into `Y`
|
|
|
|
|
1. `a` stores `true` to `X`
|
|
|
|
|
1. `a` loads `Y` (required to give `true`)
|
|
|
|
|
1. `b` loads `X` (required to give `true`)
|
|
|
|
|
- The stores precede the loads,
|
|
|
|
|
`b`’s store precedes `a`’s and `b`’s load precedes `a`’s:
|
|
|
|
|
1. `b` stores `true` into `Y`
|
|
|
|
|
1. `a` stores `true` to `X`
|
|
|
|
|
1. `b` loads `X` (required to give `true`)
|
|
|
|
|
1. `a` loads `Y` (required to give `true`)
|
|
|
|
|
|
|
|
|
|
All the places where the load was required to give `true` were caused by a
|
|
|
|
|
preceding load in _S_ of the same atomic which saw `true` — otherwise, the load
|
|
|
|
|
would be coherence-ordered-before a load which precedes it in _S_, and that is
|
|
|
|
|
preceding store in _S_ of the same atomic of `true` — otherwise, the load would
|
|
|
|
|
be coherence-ordered-before a store which precedes it in _S_, and that is
|
|
|
|
|
impossible.
|
|
|
|
|
|
|
|
|
|
## The mixed-`SeqCst` special case
|
|
|
|
|
|
SabrinaJewson
8 months ago